Heat injection process

ABSTRACT

A method for heat injection into a subterranean formation is provided. The method utilizes flameless combustion. The absence of a flame eliminates the flame as a radiant heat source and results in a more even temperature distribution throughout the length of the burner. Flameless combustion is accomplished by preheating the fuel and the combustion air to a temperature above the autoignition temperature of the mixture. Preheating hydrocarbon fuel requires the inclusion of a carbon formation suppressant such as carbon dioxide or steam to prevent carbon formation.

RELATED PATENTS

This invention is related to copending U.S. patent application Ser. Nos.897,641 and 896,864.

FIELD OF THE INVENTION

This invention relates to a method for injection of heat into asubterranean formation.

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating ofsubterranean formations of low permeability that contain oil to recoveroil therefrom. Low permeability formations include diatomites and oilshales. Formations of low permeability are not amenable to secondary oilrecovery methods such as steam, carbon dioxide, or fire flooding.Flooding materials tend to penetrate formations that have lowpermeabilities preferentially through fractures. The injected materialsbypass most of the formation hydrocarbons. In contrast, conductiveheating does not require fluid transport into the formation. Oil withinthe formation is therefore not bypassed as in a flooding process. Whenthe temperature of a formation is increased by conductive heating,vertical temperature profiles will tend to be relatively uniform becauseformations generally have relatively uniform thermal conductivities andspecific heats. Transportation of hydrocarbons in a thermal conductionprocess is by pressure drive, vaporization, and thermal expansion of oiland water trapped within the pores of the formation rock. Hydrocarbonsmigrate through small fractures created by the expansion andvaporization of the oil and water.

When the formation contains high molecular weight oil or hydrocarbonsolids, thermal conduction could also result in pyrolysis of thehydrocarbons in-situ. The products of the pyrolysis will be of lowermolecular weights and will therefore be more valuable than the originaloil. Pyrolysis of solids also creates additional voids within theformation rocks. These voids provide additional hydrocarbon mobility.

Considerable effort has been expended to develop electrical resistanceheaters suitable for injecting heat into formations having lowpermeability. U.S. Pat. Nos. 5,065,818 and 5,060,287 are exemplary, ofsuch effort. Electrical heating of formations is relatively expensivecompared to directly burning a hydrocarbon fuel. It would be preferableto provide a heat injection method in which directly burns a hydrocarbonfuel.

Gas fueled well heaters that are useful for heating formations totemperatures sufficient for ignition of in-situ fire floods aredisclosed in U.S. Pat. Nos. 3,095,031; 3,880,235; 4,079,784; and4,137,968. Provisions for the return of combustion gases to the4,137,968. Provisions for the return of combustion gases to the surfaceare not required because the combustion gases are injected into theformation. The fuel gas and combustion air also remain relatively coolas they go down a borehole toward the burner because combustion gasesrising in the borehole do not heat the burner. Additionally, a longservice life is not required due to the short time period during whichthe burner is needed. These burners are therefore not suitable for useas heat injectors and do not overcome the shortcomings of the prior artheat injector burners.

Gas fueled heaters which are intended to be useful for heat injectionare disclosed in U.S. Pat. No. 2,902,270 and Swedish Patent No. 123,137.These burners utilize flames to combust fuel gas. The existence offlames cause hot spots within the burner and in the formationsurrounding the burner due to radiant heat transfer from the luminousportion of the flame. A typical gas flame provides about a 1650° C.radiant heat source. Materials of construction for the burners must besufficient to withstand the temperatures of these hot spots. The heatersare therefore more expensive than a comparable heater without flames.The heater of Swedish Patent 123,137 would appear to result in aflameless combustion such as the present invention if the combustion airand the fuel gas were heated to a temperature above the autoignitiontemperature of the mixture. But due to the shallow depths of the heatinjection wells disclosed in that patent, the components do not appearto be heated to this extent by the combustion gases. Further, radiantheat transfer from the flames appears to be critical in obtaining thetemperature profile indicated in FIG. 2 of the Swedish patent becauselittle heat is transferred from the well bore to the formation above theborehole containing flames. Due to the existence of flames, the servicelife and the operating temperatures of these burners are unacceptablylimited.

The Swedish patent also addresses the problem of creation of carbon fromhydrocarbon gases at elevated temperatures. The carbon is removed byexchanging services between the air and the fuel lines. Any carbondeposited in a line while the line is in fuel service is removed whenthe line is in combustion air service. This requires that the fuel gaslines be as large as the combustion air lines. Because about ten molesof combustion air ar required for each mole of methane burned, andbecause combustion air is generally optimally supplied at a lowerpressure, a fuel gas line will have to be considerably larger to alsoaccommodate combustion air flow service. The burner would therefore beconsiderably more expensive than one in which services of these twoconduits are not interchangeable.

The Swedish patent also discloses that "before the gas is brought intothe gas tube . . . it can through suitable preparation in a was known inthe gas technique be given such a composition that it does not depositcoke." It is not clear what this statement means, but it likely issuggesting that hydrocarbons heavier than methane or ethane be removedby cryogenic distillation to reduce the tendency of the gas to formcoke.

U.S. Pat. Nos. 3,113,623 and 3,181,613 disclose gas fired heat injectionburners for heating subterranean formations. These burners utilizeporous materials to hold a flame and thereby spreading the flame outover an extended length. Radiant heat transfer from a flame to thecasing is avoided by providing the porous medium to hold the flame. Butfor combustion to take place in the porous medium, the fuel gas and thecombustion air must be premixed. If the premixed fuel gas and combustionair were at a temperature above the autoignition temperature of themixture, they would react upon being mixed instead of within the porousmedium. The formations utilized as examples of these inventions are onlyup to fifty feet thick and below only about fifteen feet of overburden.The fuel gas and the combustion air are therefore relatively cool whenthey reach the burner. The burner would not function at it was intendedif the formation being heated were significantly deeper.

It is therefore an object of the present invention to provide a methodto inject heat into a subterranean formation using a fuel gas combustorwhich does not require a flame in the borehole during the heatingprocess. It is a further object to provide such a method which does notrequire complicated equipment within the borehole. It is another objectof the present invention to provide a method which has a high level ofthermal efficiency.

SUMMARY OF THE INVENTION

These and other objects are accomplished by a method of heating asubterranean formation comprising at least one borehole providingcommunication from the surface to the subterranean formation to beheated, the method comprising:

combining a hydrocarbon fuel gas with a carbon formation suppressant;

passing the fuel gas and carbon formation suppressant mixture through afuel gas conduit to a mixing point within the borehole juxtapose to theformation to be heated;

passing a combustion air stream though an air conduit to the mixingpoint;

preheating either the fuel gas and carbon formation suppressant mixture,the combustion air stream or both such that the temperature of a mixtureof the streams exceeds an autoignition temperature of the mixture of thestreams;

combining the preheated combustion air and fuel gas and carbon formationsuppressant at the mixing point resulting in autoignition formingcombustion products; and

passing the combustion products through the borehole from the mixingpoint to the surface, wherein the amount of the carbon formationsuppressant combined with the fuel gas exceeds that which preventscarbon formation at the temperature of the preheated fuel gas and carbonsuppressant mixture.

Transportation of the fuel and the combustion air separately to theportion of the wellbore to be heated permits the gases to be heated to atemperature greater than the autoignition temperature of the mixture.Combining the gases at a temperature greater than the autoignitiontemperature, along with rapid mixing of the fuel with the combustionair, provides a flameless combustion. Elimination of the flameeliminates the flame as a source of radiant energy and greatlysimplifies the construction of the heater, and results in a more evendistribution of heat from the burner.

Additional fuel gas is preferably mixed with the combustion products atpoints within the borehole after the initial point of mixing. Thispermits the formation to be heated over a greater distance, and permitsadditional heat to be injected without increasing the temperature whichthe borehole equipment must be designed to withstand. Staged burningalso reduces the amount of nitrous oxide produced by providing somereburning of nitrous oxides back to nitrogen.

A carbon formation suppressant is required because the fuel gas can beheated to a temperature which favors formation of carbon fromhydrocarbons. Acceptable carbon formation suppressants include carbondioxide, water and hydrogen. Carbon dioxide and water are preferred dueto lower cost.

The flameless combustion of the present invention also results inminimal production of nitrous oxides.

Other measures to remove or prevent the formation of nitrous oxides aretherefore not required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 5 show burners suitable for use in the presentinvention.

FIG. 6 is a plot of temperature profiles along a burner at varioustemperatures.

DETAILED DESCRIPTION OF THE INVENTION

Injectors utilizing flameless combustion of fuel gas at temperaturelevels of about 900° to about 1100° C. may be fabricated from hightemperature alloys such as, for example, WASPALLOY, INCONEL 601, INCONEL617, INCOLOY 800HT, HASTELLOY 235, UNIMET 500 and INCOLOY DS. At highertemperatures ceramic materials are preferred. Ceramic materials withacceptable strength at temperatures of 900° to about 1400° C. aregenerally high alumina content ceramics. Other ceramics that may beuseful include chrome oxide, zirconia oxide, and magnesium oxide basedceramics. National Refractories and Minerals, Inc., Livermore, Calif.,A. P. Green Industries, Inc., Mexico, Missouri, and Alcoa, Alcoa Center,Penn., provide such materials.

Generally, flameless combustion is accomplished by preheating combustionair and fuel gas so that when the two streams are combined thetemperature of the mixture exceeds the autoignition temperature of themixture, but to a temperature less than that which would result in theoxidation upon mixing being limited by the rate of mixing. Preheating ofthe streams to a temperature between about 850° C. and about 1400° C.and then mixing the fuel gas into the combustion air in relatively smallincrements will result in flameless combustion. The increments in whichthe fuel gas is mixed with the combustion gas stream preferably resultin about a 20° to 100° C. temperature rise in the combustion gas streamdue to the combustion of the fuel.

Referring to FIG. 1, a heat injection well and burner capable ofcarrying out the present invention are shown. A formation to be heated,1, is below an overburden, 2. A wellbore, 3, extends through theoverburden and to the bottom of the formation to be heated. A verticalwell is shown, but the wellbore could be deviated or horizontal.Horizontal heat injector wells may be provided in formations thatfracture horizontally to recover hydrocarbons by a parallel driveprocess. Shallow low oil shale formations are examples of suchformations. In the embodiment shown in FIG. 1, the wellbore is casedwith a casing, 4. The lower portion of the wellbore may be cemented witha cement, 7, having characteristics suitable for withstanding elevatedtemperatures and transferring heat. A cement which is a good thermalinsulator, 8, is preferred for the upper portion of the wellbore toprevent heat loss from the system. A combustion air conduit, 10, extendsfrom the wellhead, 11 to the lower portion of the wellbore. A fuel gasconduit, 12, also extends from the wellhead the bottom of the wellbore.

High temperature cements suitable for cementing casing and conduitswithin the high temperature portions of the wellbore are available.Examples are disclosed in U.S. Pat. Nos. 3,507,332 and 3,180,748.Alumina contents above about 50 percent by weight based on cementsslurry solids are preferred.

Thermal conductivity of these cements can be increased by includinggraphite in the cement slurry. Between about 10 and about 50 percent byweight of graphite will result in a significant improvement in thermalconductivity. Cement slurries that contain graphite are also of asignificantly lower density than high alumina slurries and generally areless expensive than high alumina slurries. The lower density slurryenables conventional cementing of wellbores whereas heavier slurriesoften required staged cementing. Staged cementing requires considerablerig time. Graphite containing cements are not particularly strong, andare therefore not preferred when high strength is required. When asubstantial casing is utilized, high strength cement is not required andhigh graphite cement is preferred.

Choice of a diameter of the casing, 4, in the embodiment of FIG. 1 is atrade off between the expense of the casing, and the rate at which heatmay be transferred into the formation. The casing, due to the metallurgyrequired, is generally the most expensive component of the injectionwell. The heat that can be transferred into the formation increasessignificantly with increasing casing diameter. A casing of between about4 and about 8 inches in internal diameter will typically provide anoptimum trade-off between initial cost and heat transfer. The casing, 4,could optionally be provided with means to provide communication betweenthe outside of the casing and the inside of the casing after the well isbrought up to operation temperatures. Such means would relieve pressurefrom the outside of the casing. These pressures are generated byformation gases that permeate the cement. Relieving these pressurescould permit the use of casings having thinner walls. Means to providecommunication may be, for example, partially milled portions which failat operation temperatures and pressures, or plugs of aluminum orpolymers that melt or burn at service temperature and pressure. Theplugs or milled portions would serve to keep cement out of the casingwhile the casing is being cemented into place.

The fuel gas conduit contains a plurality of orifices, 13, along thelength of the conduit within the formation to be heated. The orificesprovide communication between the fuel gas conduit and the combustionair conduit. A plurality of orifices provide for distribution of heatrelease within the formation to be heated. The orifices are sized toaccomplish a nearly even temperature distribution within the casing. Anearly even temperature profile within the casing results in moreuniform heat distribution within the formation to be heated. A nearlyuniform heat distribution within the formation will result in moreefficient utilization of heat in a conductive heating hydrocarbonrecovery process. A more even temperature profile will also result inthe lower maximum temperatures for the same heat release. Because thematerials of construction of the burner and well system dictate themaximum temperatures, even temperature profiles will increase the heatrelease possible for the same materials of construction. The number oforifices is limited only by size of orifices which are to be used. Ifmore orifices are used, they must generally be of a smaller size.Smaller orifices will plug more easily than larger orifices. The numberof orifices is a trade-off between evenness of the temperature profileand the possibility of plugging.

Alternatively, air could be staged into fuel gas by providing orificesin the combustion air conduit instead of the fuel conduit.

Fuel gas and combustion air transported to bottom of the wellborecombine and react within the wellbore volume surrounding the conduits,14, forming combustion products. The combustion products travel up thewellbore and out an exhaust nozzle, 15, at the wellhead. From theexhaust nozzle, the combustion products may be routed to atmospherethrough an exhaust stack (not shown). Alternatively, the combustiongases may be treated to remove pollutants. Energy recovery from thecombustion products by an expander turbine or heat exchanger may also bedesirable.

As the combustion products rise in the wellbore above the formationbeing heated, they exchange heat with the combustion air and the fuelgas traveling down the flow conduits. This heat exchange not onlyconserves energy, but permits the desirable flameless combustion of thepresent invention. The fuel gas and the combustion air are preheated asthey travel down the respective flow conduits sufficiently that themixture of the two streams at the ultimate mixing point is at atemperature above the autoignition temperature of the mixture. Flamelesscombustion results, avoiding a flame as a radiant heat source. Heat istherefore transferred from the wellbore in an essentially uniformfashion.

The preheating of the fuel gases to obtain flameless combustion wouldresult in significant generation of carbon within the fuel gas conduitunless a carbon formation suppressant is included in the fuel gasstream. Nozzles for injection of fuel gas and oxidant suppressant areshown in FIG. 1 as 16 and 17 respectively. The carbon formationsuppressant may be carbon dioxide, steam, hydrogen or mixtures thereof.Carbon dioxide and steam are preferred due to the generally higher costof hydrogen.

Carbon is formed from methane at elevated temperatures according to thefollowing reaction:

    CH.sub.4 →C+2H.sub.2

This reaction is a reversible reaction, and hydrogen functions as carbonformation suppressant by the reverse reaction.

Carbon dioxide suppresses carbon formation by the following reaction:

    CO.sub.2 +C→2CO

Steam suppresses carbon formation by the following reactions:

    H.sub.2 O+C→CO+H.sub.2

    2H.sub.2 O+C→CO.sub.2 +2H.sub.2

The carbon dioxide and the carbon monoxide remain in equilibrium atelevated temperatures according to the shift gas reaction:

    CO+H.sub.2 O⃡CO.sub.2 +H.sub.2

When the fuel gas is essentially methane, a molar ratio of about 1:1 ofsteam to methane will be sufficient to suppress carbon formation totemperatures of about 2500° F. and a molar ratio of about 1.15:1 ofcarbon dioxide to methane is sufficient to suppress carbon formation.The molar rations of steam to methane is preferably within the range ofabout 1:1 to about 2:1 when steam is utilized as the carbon formationsuppressant. The molar ratio of carbon dioxide to methane is preferablywithin the range of about 1:1 to about 3:1 when carbon dioxide isutilized as the carbon formation suppressant. The fuel gas preferablyconsists essentially of methane due to methane being more thermallystable than other light hydrocarbons. The suppressant is additionallybeneficial because it lowers combustion rates and reduces peaktemperatures.

Referring now to FIG. 2, an alternative apparatus capable of carryingout the present invention is shown with elements numbered as in FIG. 1.In the embodiment of FIG. 2, the combustion products rise to the surfacethought a separate conduit, 19, rather than through the wellboresurrounding the air conduit, 10. The combustion product return conduitand the combustion air conduit are separate conduits connected a thebottom of the wellbore by a cross-over, 18. Fuel gas is provided througha fuel gas conduit, 12, within the combustion product return conduit,19, and the combustion air conduit, 10. Alternatively, the fuel gasconduit could be within one of the two other conduits. The combustionproduct return conduit and the combustion air conduit are cementeddirectly into the formation to be heated, 2, by a high temperaturecement, 7. If the combustion air and combustion gases conduits aresufficiently strong that they do not required significant support fromthe cement, a cement containing a high level of graphite can beutilized. This configuration should be considerably less expensive toprovide due to the absence of a large diameter casing within the hightemperature portion of the wellbore. The two smaller conduits, whenseparated laterally within the wellbore, can transfer heat into theformation more effectively than a single conduit having the same surfacearea.

The flow conduits may be made from steel, high temperature alloys suchas INCOLOY, INCONEL or HASTALLOY or ceramics, depending upon theoperating temperatures and service life desired. Ceramics are preferredas a material of construction for casings and flow conduits of thepresent invention when injection of heat at temperature levels aboveabout 1100° C. are desired.

Referring to FIG. 3, with elements numbered as in FIG. 1, a preferredembodiment utilizing metal alloy flow conduits is shown. The formationto be heated, 1, below an overburden, 2, is shown penetrated by awellbore, 3, of about twelve inches in diameter. In this embodiment, thewellbore is cased with a sacrificial carbon steel casing, 4, of abouteight inches in diameter. The casing is cemented into place using a hightemperature cement, 7, which forms an outer perimeter of the flowchannel through which combustion gases travel up the wellbore. Thecement is preferably one such as PERMACON, a high alumina cementavailable from National Refractories and Minerals, Inc. of Livemore,Calif. A combustion air conduit, 10, in this embodiment is made from analloy such as INCOLOY and is centralized within the casing. Thecombustion air conduit could be, for example, a three to four inchdiameter tube. A fuel gas conduit, 12, is centralized within thecombustion air conduit. The fuel gas conduit can be made from an alloysuch as INCOLOY and could be about three quarters of an inch indiameter. Combustion occurs in the annulus between the fuel gas conduit,and the combustion air conduit, 12. At the lower end of the formation tobe heated, within the wellbore, the combustion air conduit is incommunication with the annulus between the combustion air conduit, 10,and the casing. This annulus provides a flow path for combustionproducts to travel back up the wellbore to the surface.

The embodiment of FIG. 3 provides for conventional centralization of theflow conduits, and conventional replacement of the fuel gas line andcombustion air line if such replacement becomes necessary.

Referring now to FIG. 4, with elements numbered as in FIG. 1, apreferred embodiment of a burner is shown utilizing stacked annularshaped ceramic bricks to form a combustion gas conduit. A wellbore, 3,is shown extending into a formation to be heated, 1, under anoverburden, 2. A casing of a sacrificial material, 4, is utilized toinitially hold the ceramic bricks, 20, in place. The ceramic bricks canbe about three inches in wall thickness and each about five to ten feetin height. The bricks may be made of a high alumina ceramic material,and may be sealed together with a high alumina mortar. A combustion airconduit, 10, provides a flow path for combustion air to the lowerportion of the formation to be heated. The combustion air conduit isopen and in communication with the annulus between the combustion airconduit and the ceramic bricks near the bottom of the formation to beheated. A fuel gas conduit, 12, directs fuel gas into the volume definedby the casing in increments through orifices, 13, to provide foroxidation of the fuel gas in relatively small increments. The fuel gasconduit and the combustion air conduit may be ceramic if operatingtemperatures are to be above about 1100° C. If operating temperaturesare to be about 1100° C. or less, the flow conduits can be fabricatedfrom an alloy such as INCOLOY. The ceramic bricks are typically cementedinto place within the wellbore with a high temperature cement andpreferably a graphite containing high alumina cement.

Referring now to FIG. 5, another embodiment of a preferred heat injectoris shown with elements numbered as in FIG. 1. This embodiment ispreferred when the heat injector is to be injecting heat at temperaturesof about 1100° C. to about 1365° C. In this embodiment, the combustionair conduit, 10, the combustion gas conduit, 19, and the fuel gasconduit are all initially sacrificial materials cemented into place. Thecement is a high temperature cement. A high graphite content cement isnot preferred in this embodiment due to the lower strength of the highgraphite cements. A channel, 22, near the bottom of the formation to beheated provides communication between the combustion air conduit and thecombustion gas conduit. Communication between the fuel gas conduit, 12,and the combustion gas conduit and the combustion air conduit isprovided through conduits such as alloy tubes, 23, that may containorifices (not shown) to restrict flow of fuel gas into the larger flowconduits. Combustion of the fuel gas occurs both in the downflowcombustion air conduit, 10, an in the up flow combustion gases conduit,19. Within the formation to be heated, 1, the combustion gas and thecombustion air conduit are spaced as far apart as practical in order tomaximize the amount of heat which can be transferred to the formation atany maximum operating temperature.

The embodiment of FIG. 5 could include a ceramic fuel gas conduit or asacrificial conduit which is eliminated prior to or during initialoperation, leaving the cement defining a conduit. The sacrificialconduit may be eliminated by, for example, oxidation, melting, ormilling. A plurality of fuel gas conduits could optionally be provided.A plurality of fuel gas conduits could provide redundancy, and couldreduce the total length of tubes, 23, which are required. In theembodiment of FIG. 5, the wellbore, 3, could be of about a sixteen inchdiameter within the formation to be heated, and contain about a three tofour inch internal diameter combustion air conduit, a combustion gasconduit of about three to about four inch diameter, and one orpreferably two fuel gas conduits of about three quarters inch diameter.Orifices in the alloy tubes, 23, are sized to achieve a fuel gas flowthat would result in a nearly uniform temperature profile within thewellbore.

When ceramic materials are utilized for construction of the heatinjectors, the larger conduits (combustion air and combustion productconduits) may initially be sacrificial materials such as polymeric,fiberglass, or carbon steel. The sacrificial conduits can be cementedinto place using high alumina cements. The high alumina cement forms theconduit which remains in place after the sacrificial materials areremoved.

High alumina ceramic tubes are available that have tensile strengthsufficient to permit suspension of the conduits from a rig at thesurface. These ceramic conduits can be lowered into the wellbore assections are added at the surface. The sections can be joined by mortarand held together by sacrificial clamps until the mortar has cured. Theceramic tubes could also be held in place by sacrificial pipes untilthey are cemented into place.

Cold start-up of a well heater of the present invention may utilizecombustion with a flame. Initial ignition may be accomplished byinjecting pyrophoric material, an electrical igniter, a spark igniter,or temporally lowering an igniter into the wellbore. The burner ispreferably rapidly brought to a temperature at which a flamelesscombustion is sustained to minimize the time period at which a flameexists within the wellbore. The rate of heating up the burner willtypically be limited by the thermal gradients the burner can tolerate.

Flameless combustion generally occurs when a reaction between an oxidantstream and a fuel is not limited by mixing and the mixed stream is at atemperature higher than the autoignition temperature of the mixedstream. This is accomplished by avoiding high temperatures at the pointof mixing and by mixing relatively small increments of fuel into theoxidant containing stream. The existence of flame is evidenced by anilluminate interface between unburned fuel and the combustion products.To avoid the creation of a flame, the fuel and the oxidant arepreferably heated to a temperature of between about 1500° F. and about2500° F. prior to mixing. The fuel is preferably mixed with the oxidantstream in relatively small increments to enable more rapid mixing. Forexample, enough fuel may be added in an increment to enable combustionto raise the temperature of the stream by about 50° to about 100° F.

FIG. 6 is a plot of calculated temperatures as a function of distancefrom a mixing point in a cylindrical combustion chamber. The cylindricalcombustion chamber is of an eight inch internal diameter, and a flow of220 standard cubic feet per minute (standard at 60° F. and oneatmosphere pressure) of combustion gases containing three percent byvolume of oxygen is mixed with enough methane that combustion of themethane would increase the temperature of the stream by 10° F. Both themethane and the combustion gases are initially at the same temperature.Mixing of the methane and the combustion gases is assumed to be rapid.Heat is removed from the combustion chamber at a rate of about 375 wattsper foot of length. Temperature profiles shown as lines a through hrepresent temperature profiles for gases starting at temperatures of1400° F. through 2100° F. in one hundred degree fahrenheit respectively.

FIG. 6 demonstrates that under these conditions, the reaction betweenmethane and oxygen occurs at a useful rate. Flameless at a temperatureless than about 1500° F. would require a longer residence time than thatprovided by an eight inch diameter chamber at a low pressure in order toprovide heat to a formation at a rate greater than about 375 watts perfoot. Reaction of the fuel and oxygen is very quick at 2100° F.

EXAMPLE

Flameless combustion was demonstrated by injection of natural gas into astream of exhaust gasses in an amount sufficient for combustion to raisethe temperature of the gas stream by about 50° F. The exhaust gas streamcontained about 3.6 molar percent oxygen and was at a temperature ofabout 1550° F. The combined stream was analyzed for carbon monoxide andhydrocarbons at a point downstream of the mixing. The residence time wasabout 0.3 seconds. About 40 ppm carbon monoxide was detected in thesample but no hydrocarbons were detected. The combustion was flameless.

I claim:
 1. A method of heating a subterranean formation comprising atleast one borehole providing communication from the surface to thesubterranean formation to be heated, the method comprising:combining ahydrocarbon fuel gas with a carbon formation suppressant; passing thefuel gas and carbon formation suppressant mixture through a fuel gasconduit to a mixing point within the borehole juxtapose to the formationto be heated; passing a combustion air stream though an air conduit tothe mixing point; preheating either the fuel gas and carbon formationsuppressant mixture, the combustion air stream or both such that thetemperature of a mixture of the streams exceeds an autoignitiontemperature of the mixture of the streams; combining the preheatedcombustion air and fuel gas and carbon formation suppressant at themixing point resulting in autoignition forming combustion products;passing the combustion products through the borehole from the mixingpoint to the surface,wherein the amount of the carbon formationsuppressant combined with the fuel gas exceeds that which preventscarbon formation at the temperature of the preheated fuel gas and carbonsuppressant mixture.
 2. The method of claim 1 wherein the fuel gas andcarbon formation suppressant mixture and the combustion air are bothpreheated by heat exchange with the combustion products flowing from themixing point to the surface.
 3. The method of claim 2 wherein additionalpreheated fuel gas and carbon formation suppressant is mixed with thecombustion products in the borehole between the mixing point and thesurface at points that are juxtapose to the formation to be heated. 4.The method of claim 2 wherein the fuel gas consists essentially ofmethane.
 5. The method of claim 4 wherein the fuel gas and carbonformation suppressant mixture and the combustion air are both heated toa temperature above about 1000° F.
 6. The method of claim 5 wherein thecarbon formation suppressant comprises water and the mole ratio of waterto carbon in the stream is greater than about
 1. 7. The method of claim5 wherein the carbon formation suppressant comprises carbon dioxide andthe mole ratio of carbon dioxide to fuel gas carbon is greater thanabout
 1. 8. The method of claim 5 wherein the carbon formationsuppressant comprises a mixture of carbon dioxide and water. .Iadd.
 9. Amethod of supplying heat using a flameless combustor, the methodcomprising:combining a hydrocarbon fuel gas with a carbon formationsuppressant; passing the fuel gas and carbon formation suppressantmixture through a fuel gas conduit to a mixing point within theflameless combustor; passing a combustion air stream though an airconduit to the mixing point; preheating either the fuel gas and carbonformation suppressant mixture, the combustion air stream or both suchthat the temperature of a mixture of the streams exceeds an autoignitiontemperature of the mixture of the streams; and combining the preheatedcombustion air and fuel gas and carbon formation suppressant at themixing point resulting in autoignition forming combustionproducts,wherein the amount of the carbon formation suppressant combinedwith the fuel gas exceeds that which prevents carbon formation at thetemperature of the preheated fuel gas and carbon suppressant mixture..Iaddend.